1. Field of the Invention
The invention relates to a method of recovering the modeled state of a system.
2. Description of Related Art Including Information Disclosed under 37 CFR 1.97 and 1.98
In many electronic systems it is necessary to provide a framework to facilitate the storage of state information derived from monitoring entities within an external system. An example where this might be required is in a transaction based system with a high request inter-arrival rate such as that of monitoring a number of users logged on to a computer network.
In such systems, the problems of modeling the system and the state of the system are non-trivial. One approach that is used is to model the system using state counters associated with individual entities within the system, representing for example, the usage of the entity at any one instant in time. The required operations on these counters simply comprise increment and decrement requests.
Typically, in transaction based systems which have a high request inter-arrival rate, the option of storing this data in non-volatile memory can be dismissed due to the lengthy access times. Therefore volatile memory is the only option. However, in using volatile memory there is the risk that all state information will be lost in the case of a system failure of the monitoring system.
One known approach to the recovery of state information in a transaction based system, where one or more counters represent the correct, instantaneous state of the monitored system, when a failure of the monitoring application occurs and the state counters are lost, is to reset all the counters to zero, implicitly accepting that all the previous state data is lost and unrecoverable. The monitoring operation then continues as before and the counters are adjusted in response to incoming transaction requests.
The known approach suffers from the problem that the system will receive decrement requests which relate to the initial state before failure. However, the appropriate counters should not be decremented as they will tend to a negative value, thus misrepresenting the correct, instantaneous state of the monitored system. This occurs since there will be decrement requests received relating to increment requests received during the initial state. The problem with this is that the current state counters have no knowledge of what the state of the system was in the initial state. Therefore, any decrement requests received relating to increment requests received during the initial state will be deducted from the current state counters incorrectly.